When there is a fire breakout in the room the temperature increases. This ultra compact and low cost fire alarm senses fire breakout based on this fact.

Transistor BC177 (Q1) is used as the fire sensor here. When the temperature increases the leakage current of this transistor also increases. The circuit is designed so that when there is an increase in the leakage current of Q1, transistor Q2 will get biased. As a result when there is a fire breakout the transistor Q2 will be on. The emitter of Q2 (BC 108) is connected to the base of Q3(AC 128). So when Q2 is ON Q3 will be also ON. The transistor Q3 drives the relay which is used to drive the load ie,light,bell,horn etc as an indication of the fire. The diode D1 is used as a free wheeling diode to protect it from back EMF generated when relay is switched.

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Circuit diagram with Parts list.

Notes.

The Preset R1 can be used to desired temperature level for setting the alarm ON.

This is not a latching alarm,that is;when the temperature in the vicinity of the sensor decreases below the set point the alarm stops.

The circuit can be powered using a 9V battery or a 9V battery eliminator.

All capacitors are electrolytic and must be rated at least 10V.

The load can be connected through the C,NC,NO points of the relay according to your need.

The calibration can be done using a soldering iron, and a thermo meter. Switch ON the power supply.Keep the tip of soldering iron near to the Q1. Same time also keep the thermometer close to it. When the temperature reaches your desired value adjust R1 so that relay gets ON.Done!

hi
I was going through this ckt diagram and I have few queries. Can you help in sorting out these

1) its mentioned that The emitter of Q2 (BC 108) is connected to the base of Q3(AC 128). but I think as it shows it is npn transistor so according to diagram collector is connected to the base of Q3
2)And we applying 9V dc so I just want to know the polarity of the battery like the positive terminal is towards Q1 or Q3 because the emitter of Q1 and collector of Q3 they both should have more POsitive compare to base to conduct .
can any body reply as soon as possible

NO means normally open contact(when relay is de-energised this contact will be open when the relay in energised it will become close contact) Similarly NC means normally closed contact will go open when relay is de-energised. R1 os a variable resistance which can be varied from 0 ohms to 470 kilo ohms as per required temperature level. It will be a practical adjustment

AC128 is wrongly shown as NPN transistor it should be pnp and its emitter should go to + and collector should go to the relay. You can use any switching pnp transistor like S8550, 2N2905, BD136. BC557 cannot give the required drive current.

Hi Alok yes you have to leave base open as the temperature increases its collector emitter leakage current will increase and that will trigger the alarm. The basic principle used here is the CE leakage current vs temperature characteristic.

Hi, am in Nigeria and I appreciate this project… Any mini soundless generator project available? I will use this project of yours as 1st alternative project but I need more just incase my supervisor doesn’t approve it. Thanks

Hi,
If I am not wrong, the potentiometer here adjusts the sensitivity of the circuit. Since it biases the transistor Q2 from the output of Q1. It means that, by adjusting this potentiometer (a variable resistance) you can change the sensitivity of the circuit (not full volume). For example, if the circuit is made more sensitive, it will go on for a very less amount of fire etc.

Hi guys.. the circuit may work but take note you can’t use it as a standard fire alarm in rooms especially ventilated one,you need to heat the room first or the room is already on flames to activate the alarm and you can’t control the fire anymore. For kitchen you can used heat sensing alarm like this and other fire alarm circuits using heat sensing component such as thermistor where smoke alarm will be not suitable but for rooms use smoke alarm which is cheaply available for safety reason.

Sir,
I am studying the circuit diagram and I got confused.
Is it the collector of Q2 will be connected to the base of Q3? (It says on the text :The emitter of Q2 (BC 108)is connected to the base of Q3(AC 128).)
Or is this a typographical error?
Please help.

Hi Sarah for fire alarm DC is preferable, so that they can work during power cut also. You can use rechargeable batteries with a suitable charger for 9 volt. this will be fail safe, whenever power supply is available it will be charging the battery as well powering the alarm. during power failure the battery will take over.

sir i tried your ckt…but after lots of attempts…still the ckt is not responding to the heat.when i connect to the supply the relay gets on (showing that current is flowing through the ckt)..but after that it doesnot respond to heat at all.please help!

This circuit will not work properly as soon as you plug it. You need to calibrate the circuit before using. Calibration can be done by adjusting the POT R1.
I tested this circuit and there is no problem with it.

sir i am from bangladesh.your circuit is very easy to establish.but i face a problem to find transistor Q1(bc177).its unavailable.equivalents of this transistor bc178,bc179 are also unavailable.please help me to find another transistor.

hi sir..
im quennie
from the philippines and
i just want to ask if you have
a diagram of the ic you used..
i mean if you have some diagram
to guide on so that i can place the materials in their
right position and place..

sir the circuit u provide for fire alarm is not working according to the circuit.
actully I am a student and working on this fire alarm project. I joined all the connections according to ur circuit but it is not working plz help me…
at place of Q3(AC128)i used another transister like the same of this.
plz sir help me to complete my project ..
waiting for reply..
thank you.